Pes planus, or pes valgo planus, is a deformity producing a severe flat foot. The deformity occurs largely at one particular joint, the talocalcaneal articulation, which is the joint between the talus and calcaneus bones in the foot. There are typically three separate components of a valgus deformity at this joint: first, the calcaneus has a valgus position; second, the head of the talus angulates downward; and third, the forefoot is totally abducted in relation to the hind foot. In addition, the Achilles tendon may be pulled laterally due to the outward rotation, or eversion, of the calcaneus.
Pes valgo planus often results from the failure of the arch to form in one or both feet of a child aged two to four, which is the typical age for the natural formation of the arch in the foot. The arch may fail to form due to loose joint connections or baby fat lodged between the foot bones of the child. Pes valgo planus also occurs in adults as a result of Posterior Tibial Tendon Dysfunction (PTTD), one of the more common tendon disorders involving the ankle. The posterior tibial tendon helps support the arch of the foot and provides power to point the foot down and to turn the foot inward. PTTD is typically caused by chronic inflammation, degenerative changes, and occasionally trauma, which lead to stretching, laxity, and eventual rupture of the posterior tibial tendon. People suffering from PTTD often experience tenderness and inflammation along the inner part of the ankle, and may experience weakness when standing on their toes. As the disease progresses, the person may experience loss of the arch of the foot while standing, and the foot tends to turn outward under weight. Late stages of the disease are associated with a flat foot deformity with degenerative changes in the joints below the ankle.
In many cases, the symptoms of pes valgo planus may be treated using conservative measures such as anti-inflammatory medications, rest, ice, shoe inserts or orthotic supports, or even ankle-foot braces. However, in some cases, such measures prove inadequate and the person may continue to experience severe foot or ankle pain or suffer from night cramps, pain when walking and/or standing, or lower back and knee pain. In such cases, a subtalar implant may be used to correct the flatfoot deformity while maintaining mobility of the subtalar joint. The subtalar implant is a small device that is inserted into a small opening in the talocalcaneal joint called the sinus tarsi. The placement of the implant restores the arch by preventing the displacement of the talus and by preventing the foot from rolling-in (pronating). In some cases, tissue may grow around the implant which helps hold the implant in place within the sinus tarsi.
Foot size varies between individuals. Different levels of correction may be required based on foot size and severity of condition. To accommodate these considerations, subtalar implants may come in a variety of sizes. To determine the proper sized implant for a particular person, trial implants may be implanted and removed until the proper size is found for the person.
An example of an existing trial system includes trial implants with threaded holes in the ends that correspond to male threads on the end of a driver handle. A reversed threaded locking device may be included on the driver handle. The reverse threaded locking device may be turned until it jams against the trial implant to lock the trial implant onto the threaded end of the driver handle stem. This may cause problems as the trial implant may not properly lock onto the end of the driver handle. Improper locking may result in the trial implant remaining in the patient when the driver handle is rotated to remove the trial implant.
Another trial system utilizes a “lollipop” design. In this design, each trial implant is permanently mounted to a driver handle. The trial may be implanted and removed using the one piece driver handle. The entire tool would then need to be sterilized or properly disposed. Furthermore, a driver handle is required for each trial implant that is tested. This system can be expensive and bulky.